But people take notably different approaches to this form of exercise. Some complete only one sustained, all-out, four- or five- minute bike ride or sprint — a single interval — and then are done. Others practice standard interval training, involving repeated brief bursts of almost unbearably taxing exertion, interspersed with restful minutes of gentler exercise. Some people perform such sessions two or three times per week; others almost every day.

Video

The 20-Minute Workout

Gretchen Reynolds, the Phys Ed columnist, on the science of high-intensity interval training, or H.I.I.T., which scientists are finding can be as effective as longer endurance training.

The science of intensive interval training has, though, been lagging behind the workout’s popularity. Past studies of HIIT, as the practice is commonly known, had established that as measured by changes in cellular markers, standard short-burst HIIT training may improve aerobic fitness up to 10 times as much as moderate endurance training. But scientists had not determined whether a single sustained interval likewise improves fitness, or the ideal number of HIIT sessions per week.

So to clarify those issues, researchers at two of the laboratories most noted for HIIT science set out to learn more about the best way to do interval training.

First, for a study published this month in Experimental Physiology, scientists at McMaster University in Ontario gathered 17 healthy young men and women and divided them into groups. Ten of them were asked to exercise on two separate days. On one day they completed a standard HIIT session consisting of four 30-second bouts of all-out, tongue-lolling effort on a stationary bicycle, alternating with four minutes of recovery between. On another day they completed a single uninterrupted interval lasting for about four minutes, by which time each rider had combusted the same amount of energy as during the stop-and-go session. Before and after the workouts, the scientists gathered blood and muscle samples.

Separately, the remaining seven volunteers did the continuous four-minute workout three times a week for six weeks. The researchers again collected blood and muscle samples, and monitored changes in the riders’ athletic performance by having them ride as hard as possible for a specified period of time.

When collated and compared, the data showed that the physiological differences among the two groups of riders were notable and, in some ways, strange.

On the one hand, the scientists found no significant variations in how the muscles of riders in the first group responded to a single session of interval training, whether of the standard stop-and-go variety or a sole sustained effort. In both cases, the riders showed immediate, post-exercise increases in their blood levels of certain proteins associated with eventual improvements in endurance capacity.

But when the researchers checked blood and muscle tissue in the second group of riders after they had completed six weeks of single-interval training, some of the pending improvements seemed to have evaporated. These riders’ muscle tissues now had only average — not augmented — amounts of the chemicals that help cells to produce more energy, a reliable marker of fitness. This finding was in stark contrast to the results of earlier work by the same researchers, in which they found that six weeks of standard short-burst HIIT exercise resulted in significant, sustained gains in these markers.

The implications of the new study are not altogether clear, said Martin Gibala, the chairman of the department of kinesiology at McMaster University and senior author of the study, but “it would appear,” he said, “that there is something important, even essential, about the pulsative nature” of on-off HIIT training if you wish to reap sustained physiological improvements.

In more practical terms, before you riff on your current workout, check to see whether reliable science supports your improvisation.

That caution is underscored by the results of the other major new study of interval training, this one published this month in PLOS One and undertaken at the Norwegian University of Science and Technology in Trondheim, Norway. In it, scientists asked volunteers to perform a total of 24 standard HIIT sessions over either three or eight weeks, meaning that the volunteers exercised either three times per week or almost every day and sometimes twice on the same day.

At the end of the prescribed time, those who had completed three HIIT sessions per week had improved their endurance capacity by almost 11 percent. But those exercising daily displayed no such improvements and, in some, endurance declined. Only after those volunteers had quit training altogether did their aerobic capacity creep upward; after 12 days of rest, their endurance peaked at about 6 percent above what it had been at the start, suggesting, the researchers believe, that daily high-intensity interval sessions are too frequent and exhausting. In that situation, fatigue blunts physical adaptations.

The takeaway of both studies is that it is best, if you wish to perform high-intensity interval training, to stick to what is well documented as effective: a few sessions per week of 30- or 60-second intervals so strenuous you moan, followed by a minute or so of blessed recovery, and a painful repetition or four. Done correctly, such sessions, in my experience, get you out of the gym quickly and inspire truly inventive cursing.